The subject invention relates to industrial controls. It finds particular application with the transmission and serialization of machine safety control signals, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Machine control employs digital and/or analog communication to exchange information across various networks. Typically, various protocols are employed with such networks to facilitate communication between a controller(s) and one or more control components such as I/O modules, drives, servos, switches, sensors, etc. Conventional protocols accept data from a source (e.g., controller) and package the data for transmission to one or more data recipients (e.g., control components). Such protocols have limited communication and typically are suitable only for certain layers within an automation pyramid and/or limited in their functionality (e.g., single master system running a master/slave protocol). This may result in barriers within the automation architecture that are difficult to penetrate and that require complex bridging devices without being able to fully bridge the gap between the various systems that are quite different in nature.
Machine control typically includes various safety devices and associated controls that can be used to mitigate machine accidents that can harm an individual. Conventional systems typically hardwire pushbuttons (e.g., an e-stop) directly into a controller to handle emergency situations which could result in harm to an individual. In this manner, a user can shut down an operation by pressing the appropriate button. Safety signals can also be sent through various communication channels within a control system to prevent a potentially dangerous condition from occurring. Thus, maintaining the integrity of such safety signals is critical to insure that appropriate measures are taken in the event of an emergency.
A conventional safety network is designed to detect errors and react with pre-determined safe operation. Typically, this means placing the output signals in a state which would cause the machine to stop. The protocol within a safety network takes measures to ensure a high level of integrity within the application. These measures, such as message redundancy and cross-checking, ensure that safety messages are reliably transmitted from one device and received at another in a predetermined time and with the integrity of the data content maintained or that the system goes to a predetermined safety state.
In conventional machine control architectures, safety data is transmitted in a parallel fashion in order to ensure data integrity via hardware and/or software redundancy. In one example, each transmission path includes one or more isolation elements such as optical couplers, magnetic couplers, fiber optics, isolators, etc. to isolate the machines from the control systems. Such isolation elements can be unreliable and compromise data transmission and quality. In addition, each isolation element can draw additional current which leads to excess power consumption. Surplus heat can be introduced to the control system as a result of such additional current draw. Moreover, each isolation element can take up a portion of limited space available in today's solid state control devices.
Conventionally, isolation elements increase multiplicatively in relation to the number of data channels. In one example, a control system with four output channels would require eighteen isolators to maintain data integrity. In another example, a control system with sixteen output channels would require sixty-six isolators to maintain data integrity. From a design standpoint, such a large number of isolation elements can make board layout difficult, increase bus traffic, draw excessive current, increase heat, lower reliability, shorten product life, drive up system cost and compromise optimal design methodologies and system configuration.
FIG. 1 illustrates a typical industrial control system with two output channels. In this embodiment, a main microprocessing processor unit (MPU) 10 and a peer MPU 12 are employed to process data in a redundant manner. The main MPU 10 receives status and provides power and control via five data channels, (e.g., two for status, two for control, one for power) to an output component 14. At substantially the same time, the peer MPU 12 receives status and provides power and control via five different data channels to an output component 16.
Each respective data channel can be turned off by removing power from the output circuitry. The main MPU 10 can shut down 24 VDC power via a power shut off component 18. The peer MPU 12 can shut down the power via a common line through a common shut-off component 20. Data is transmitted to two output channels, a source output screw terminal 22 and a sink output screw terminal 24. The output component 14 is coupled to the source output screw terminal 22 and the output component 16 is coupled to the sink output screw terminal 24.
In order to achieve signal isolation in this conventional two channel industrial control architecture, one or more isolation elements are employed with each data channel. As shown, five isolation elements are employed with each of the processing units, 10 and 12. Thus, at least ten isolation elements are required in this control system to provide two output channels. This multiplicative ratio of output channels to isolators can have deleterious effects in design and application of the control system, as mentioned above.
What are needed are systems and methods that permit flexible machine control architecture to reliably transfer data. Eliminating the multiplicative ratio in the number of isolation elements allows flexibility to solve these application problems effectively.